It is now possible to approach the CNS with experiments designed to correlate the physiology and morphology of synaptic transmission as has been accomplished, for example, at the neuromuscular junction by using the method of intracellular injection of horseradish peroxidase (HRP) into both pre- and postsynaptic neurons. Labelled neural elements can be easily identified in the light and electron microscope, thus physiologically identified synapses can be investigated with ultrastructural methods. The proposed studies will take advantage of this approach to investigate the ultrastructural changes occurring in identified presynaptic axons associated with transmitter release and the recovery of these changes, and to investigate the spatial arrangements of synapses made by these axons on identified postsynaptic neurons. The arrangements of synapses made by these axons on identified postsynaptic neurons. The isolated lamprey spinal cord will be used for these investigations. Quantitative measurements from electron micrographs of synapses from stimulated and control preparations will be made using an interactive computer graphics terminal interfaced with a digital computer. The location of identified synaptic terminals will be estimated with a purely physiological method based on measurements of the membrane time constant, the electrotonic length of the dendritic tree, and the dendritic to soma conductance ratio. These estimates will be compared with the locations determined morphologically.